1. Field of the Invention
This invention relates to starting powders for producing ITO sputtering targets that are useful in the formation of transparent, electrically conductive films, as well as sintered ITO. The invention also relates to processes for producing such starting powders and ITO sinters.
2. Background Information
Indium-tin oxide (ITO) sputtering targets are used in the formation of transparent, electrically conductive films. Such targets are customarily produced by sintering a compact of a tin oxide containing indium oxide powder which is either a mixture of indium oxide and tin oxide powders or derived from a coprecipitated powder. Examples of the starting materials for ITO production include an indium oxide powder, a tin oxide powder, a mixture of indium oxide and tin oxide powders, and a tin oxide containing indium oxide powder derived from a coprecipitated powder. To prepare an indium oxide powder, an aqueous solution of an indium salt as a starting material for synthesis is reacted with aqueous ammonia or an alkali hydroxide such as sodium hydroxide to make indium hydroxide, which is washed with water, dried and sintered. If a tin salt is incorporated in the aqueous indium salt solution, a tin-containing coprecipitated powder will result and from this powder, a tin oxide containing indium oxide powder can also be obtained.
The tin content of ITO is variable with the conditions of film formation and the desired film characteristics; typically, it ranges from 2 to 20 wt %, particularly from 5 to 15 wt %, in terms of SnO.sub.2.
Various methods have been proposed in association with the preparation of the starting powders for ITO production. Unexamined Published Japanese Patent Application No. 45697/1979 teaches a method in which succinic acid and urea are added as precipitants to thereby produce a powder having improved dispersibility; Unexamined Published Japanese Patent Application No. 7627/1987 teaches a method in which a precipitate is washed with an organic solvent such that a dried powder will have improved dispersibility; Unexamined Published Japanese Patent Application No. 199862/1988 teaches a method for direct oxidation of an In-Sn alloy; unexamined Published Japanese Patent Application No. 21752/1987 teaches a method in which a starting powder is heat treated at high temperature to thereby remove the volatile components, followed by sintering; Unexamined Published Japanese Patent Application No. 290527/1989 teaches a method in which the water in a precipitate is replaced by an organic solvent in order to produce a dried powder having improved dispersibility; Unexamined Published Japanese Patent Application No. 212787/1989 teaches an indium formate powder as a starting material for ITO film; Unexamined Published Japanese Patent Publications No. 193939/1993 and No. 29770/1995 and Examined Japanese Patent Publication No. 68935/1994 teach methods for producing high-density sinters from powders having specified characteristics; Examined Japanese Patent Publication No. 42109/1995 teaches a method for producing a high-density sinter under specified conditions for precipitation which is effected during synthesis of a powder; and Unexamined Published Japanese Patent Application No. 148638/1993 teaches a method in which the particle size of a starting powder and the oxygen pressure for sintering are adjusted such as to produce a high-density sinter.
Other proposals for the production of high-density sinters include hot pressing, HIP (hot isostatic pressing) and other methods in which pressure is applied under hot conditions to allow for an increase in the density of the sinter being formed.
A problem with the prior art is that when hydroxides are heat treated to be converted to oxides by the method of powder synthesis using organic reagents, lower oxides of indium and tin may potentially form. In addition, with a view to producing high-density sinters, Unexamined Published Japanese Patent Application No. 193939/1993, Examined Japanese Patent Publication No. 29770/1995 and Examined Japanese Patent Publication No. 68935/1994 specify the characteristics of powders such as the specific surface area as measured by the BET method and the average particle size; however, none of these patents refer to the presence and possible effects of substances such as halogens and lower oxides (e.g. tin oxide) that may potentially evaporate during sintering. The methods of producing high-density sinters by applying increased pressure under hot conditions as by hot pressing or HIP, as well as the methods so far proposed for preventing the formation of lower oxides by performing sintering in a pressurized oxygen atmosphere have had the following difficulties:
(1) the volatile components in the oxide powder will evaporate during sintering to cause weight loss or produce pores in the sinter being formed, thereby reducing its density; and PA1 (2) hot pressing, HIP and the sintering in pressurized oxygen require more investment than normal sintering and hence involve increased manufacturing costs, thereby imposing constraints on the effort to produce larger targets.